As a Ph.D. candidate for Hakk? Ögelman, I have become a scientifically
productive and collaborative X-ray astronomer, focusing on the interaction
between isolated pulsars and their surroundings. Highlights of my graduate
research include studies of:

the Vela pulsar;

the conversion of ``spindown power'' into a pulsar outflow;

X-ray observations of a jet from the Vela pulsar;

radio observations of the Vela jet interaction; and

high resolution X-ray imaging of the Vela pulsar wind interaction.

My graduate career has provided several valuable experiences, in collaboration
with researchers in the field and as graduate student PI on ROSAT
and ASCA projects.

The VELA PULSAR and PULSAR SPINDOWN

I have concentrated on the Vela pulsar and its supernova remnant (SNR)
for my thesis, but the lessons learned are applicable to the general problem
of pulsar spindown and wind nebulae. It is clear that isolated pulsars
-- including Vela -- continuously lose rotational energy, but that only
a small fraction of the spindown power is accounted for by direct radiation
from the pulsar. The general theoretical consensus is that the remainder
leaves the pulsar via a relativistic pair-plasma and magnetic wind, but
the precise details of the wind, such as its composition and generation
are poorly understood. The study of pulsar outflows is important because
it tests physics at the extreme, since pulsars are laboratories which sustain
intense magnetic and gravitational fields that cannot be found on earth.
The streaming wind particles do not radiate, so to probe the outflows we
must find the signatures of their interaction with the ambient medium.
The Vela pulsar is ideal for such studies because it is one of the nearest
young pulsars active from radio to &gamma;-rays
and has a low absorbing neutral hydrogen column density.

The VELA JET

The Vela jet appears in 0.7-2.4 keV ROSAT X-rays as a 45 arcmin
elongated structure (Figure 1). Diffuse X-ray emission had been previously
detected from the region southwest of the pulsar by EINSTEIN (Harnden
et.al. 1985, ApJ 299, 828), but the ROSAT\ observation is the first
to reveal that the feature is narrow, collimated, and remarkably symmetrical.
Other researchers missed the jet structure in the ROSAT band because
the soft X-ray SNR emission below 0.7 keV dominates the jet in a broad-band
image. When I generated a narrow-band image the structure is strikingly
obvious. Since the morphology of the 12 arcmin wide (FWHM) structure
is center-filled and not shell-like, and the spectrum is consistent with
thermal emission, I interpret the structure to be a ``cocoon'' of hot plasma
formed by a pulsar jet interacting with the ambient SNR material. A simple
kinematic model of this interaction which I constructed shows that the
mechanical power required to drive the jet is a significant fraction of
the pulsar's spindown power of 7 1036 erg s-1,
implying that it is indeed the result of a pulsar-driven outflow.

I continue the study of the jet with observations by ASCA and
the ROSAT High Resolution Imager (HRI). My extracted ASCA
spectrum, of much higher resolution than the ROSAT spectrum, showed
that the ``tip'' of the cocoon -- the point where the jet should be interacting
with the SNR -- has two spectral components. There is a low-temperature
thermal component (0.3 keV) and a higher
energy continuum which extends to at least 7 keV. The kinematic parameters
of the cocoon model are not altered significantly. This work is submitted
for publication, and additional observations are scheduled which should
complete the ASCA coverage of the cocoon (as a test for spectral
variations along its length). My project to map the cocoon with the HRI
will also be completed within the coming ROSAT\ observing cycle.

Figure: X-ray (left; 0.7-2.4 keV) and radio (right; 90 cm)
images of the Vela X region. The X-ray jet is center filled, while
the radio emission appears to form a ``sheath'' around the X-ray jet. The
pulsar is at coordinate (0,0), measured in arcminutes.

VLA OBSERVATIONS of VELA JET

We were very excited about the Vela jet, and embarked on a collaboration
with Dale Frail and Michael Bietenholz to do a matched-resolution study
of the radio emission in the vicinity of the pulsar. At the time of our
1995 Nature paper, we knew that the maximum of radio emission --
Vela X -- was concentrated near the termination point of the X-ray cocoon.
I performed the initial data reduction of the 330 MHz data and had
a role in the revision of the manuscript, which is accepted for publication
in ApJ. The new VLA maps that we produced show that in radio there is a
bright filament which traces from the pulsar southward to the center of
Vela X. The geometry is reminiscent of a cylindrical sheath or shell around
the X-ray cocoon (see Figure 1). At present it is not clear whether
the radio emission is produced by shocked electrons accelerated in situ
or by the compressed pulsar wind.

VELA COMPACT NEBULA

The Vela pulsar has been known to have a 2 arcmin
diameter hard X-ray compact nebula surrounding it since a 1985 observation
with EINSTEIN, but its morphology was not fully resolved. Hakk?
and I are working jointly with Fred Seward using the ROSAT HRI to
image the nebula and perform pulsar timing. While Fred has concentrated
on the timing aspect, I have constructed a very deep X-ray image of the
nebula with 4 arcsec resolution. It is clear now that the nebula has
the shape of a bow shock and is not spherically symmetric as is
classically envisioned (Figure 2). The position angle of the bow shock
apex is consistent with the observed optical proper motion of the pulsar
(as determined by myself, using HST observations, and others). I conclude
that the pulsar must be moving supersonically through the ambient medium,
producing a spherical wind which interacts at
a shock front to form the characteristic bow shock shape. This is perhaps
the most exciting part of my work because it should provide a direct probe
of pulsar wind physics. This is work in progress -- one more HRI observation
has been obtained -- and I expect it to be a very fruitful area of research.

Figure: False greyscale ROSAT HRI image of the Vela pulsar
and X-ray compact nebula. The nebula has the shape of a bow shock, and
is consistent with the independently determined proper motion of 100 km s-1
to the northwest (upper right). The pulsar is at coordinate (0,0), measured
in arcseconds. The false greyscale is use to increase contrast; in reality
the intensity decreases monotonically from the pulsar.